1. Field of the Invention
The present invention relates to atomic magnetometers and, more particularly, to a micro-fabricated atomic magnetometer and a method of forming the magnetometer.
2. Description of the Related Art
An atomic magnetometer is a device that measures the strength of a magnetic field by determining a frequency known as the Larmor frequency. The Larmor frequency, in turn, is the frequency of the magnetic moment of a contained group of in-phase, spinning, outer shell electrons of alkali atoms moving in precession in response to the magnetic field. A magnetic field strength B is defined by the equation B=hvL/γ, where h is Plank's constant, hvL is the Larmor frequency, and γ is the gyromagnetic ratio (e.g., 7 Hz/nT for 87Rb and 3.5 Hz/nT for Cs).
FIG. 1 shows a block diagram that illustrates an example of a prior art atomic magnetometer 100. As shown in FIG. 1, atomic magnetometer 100 includes a vertical cavity surface emitting laser (VCSEL) 110, and an optics package 112 that lies above VCSEL 110. Further, atomic magnetometer 100 also includes a vapor cell 114 that lies above optics package 112, and a photo detector 116 that lies above vapor cell 114.
In addition, vapor cell 114 contains a gas 118 that includes alkali atoms, which have a single electron in the outer shell, and buffer atoms, which reduce collisions between the alkali atoms and the inner surface of vapor cell 114. For example, vapor cell gas is commonly implemented with alkali atoms such as 85Rb atoms, 87Rb atoms, K, and Cs atoms, and buffer atoms such as N2. Further, atomic magnetometer 100 can optionally include a lower coil 120 and an upper coil 122 that lie below and above vapor cell 114.
In operation, VCSEL 110 outputs light which is attenuated and circularly polarized by optics package 112. The circularly polarized light output by optics package 112 is then directed into vapor cell 114. The light output by VCSEL 110 is tuned to a frequency which, when circularly polarized, is absorbed by the single electrons in the outer shells of the alkali atoms in the gas 118 contained within vapor cell 114.
For example, VCSEL 110 can be tuned to output light with a wavelength of 794.8 nm which, after being circularly polarized, is absorbed by the single electrons in the outer shells of the 87Rb atoms. VCSEL 110 can alternately be turned to output light with a wavelength of 894.35 nm which, after being circularly polarized, is absorbed by the single electrons in the outer shells of the Cs atoms.
When the single electron in the outer shell of an alkali atom absorbs light energy, the electron transitions to a higher energy level, and then falls back to one of a number of energy levels (Zeeman sublevels within the hyperfine energy levels) that are associated with the outer shell. The quantum selection rules define exactly which state the electron will result in. If the electron absorbs right hand circularly polarized light, then the electron rises to a higher energy level, while the projection number M of the electron is raised by +1.
When falling back, the electron emits a photon in a random direction, and always falls back to the highest energy level that is associated with the outer shell. In addition, when the electron falls back, the projection number M of the electron also changes by −1, 0 or +1 in a random manner.
Thus, if a number of such events occur to the same electron, each time the electron goes to a higher state, the projection number M of the electron is always +1. However, on average, as the electron falls down to the ground state, the change in the projection number M of the electron is zero. As a result, the electron will eventually land on the highest M level in the ground state. In the gas under consideration, both the ground state S1/2 and the elevated P1/2 (or P3/2) state have the same number for M levels. Thus, when the electron reaches the highest M level in the ground state, the electron cannot be pumped because there is not a higher M level in the excited state.
To again reabsorb light, the population in the ground state M levels has to be depumped. Additional energy (magnetic or optical) must be supplied to the electron at a frequency called the Larmor frequency. The additional energy at the Larmor frequency causes the electron in the highest ground state M level to drop to a lower M level that is associated with the outer shell where the electron can again absorb light energy.
Thus, the photons that pass out of vapor cell 114 into photo detector 116 include a non-absorption component, which represents the light output by VCSEL 110 that was not absorbed by the electrons in the outer shell of the gas 118 within vapor cell 114, and an emission component, which represents the photons that are randomly emitted by the falling electrons. Photo detector 116 detects these photons, and generates an output signal that has both a non-absorption component and an emission component.
Two of the common approaches to adding additional energy at the Larmor frequency are the Bell-Bloom (BB) technique and the MX technique. In the BB technique, the light output by VCSEL 110 is modulated by a frequency that is swept across a range of frequencies. When the light output by VCSEL 110 is frequency modulated at the Larmor frequency, the electrons drop to a lower energy level and begin reabsorbing light energy, which causes a noticeable dip in the intensity of light received by photo detector 116. Thus, the Larmor frequency can be determined by determining the modulated frequency that caused the intensity of the received light to dip.
In the MX technique, an RF signal is applied to the lower and upper coils 120 and 122 to create an alternating magnetic field that is aligned with the longitudinal axis of the light emitted by VCSEL 110, while the frequency of the RF signal is swept across a range of frequencies. When the frequency of the RF signal becomes equal to the Larmor frequency, the electrons drop to a lower energy level and begin reabsorbing light energy, which causes a noticeable dip in the intensity of light received by photo detector 116. Thus, the Larmor frequency can be determined by determining the RF frequency that caused the intensity of the received light to dip.
Two of the drawbacks of conventional atomic magnetometers are size and cost, which then limit the types of applications where atomic magnetometers can be commercially utilized. Thus, there is a need for micro-fabricated atomic magnetometers which can be mass produced in conventional integrated circuit fabrication facilities, thereby reducing both size and cost and significantly increasing the types of applications where atomic magnetometers can be commercially utilized.